Understanding how purification plants work is fundamental to manage them in the best possible way, for this reason BOD5 measurement is a very useful tool to verify the conformity of wastewater discharges from both urban and industrial purification plants.
It is worth remembering that the Biochemical Oxygen Demand, BOD5, is the amount of oxygen required by a microorganism in a specific number of days to oxidize in the dark and at 20 °C the organic substances present in a liter of water. It therefore helps to understand the amount of organic pollutants that the water itself contains. BOD5 detects the amount of oxygen consumed in 5 days. There are many technologies used to measure this quantity: with the physical-chemical method, the most frequent, a sample is taken and a dissolved oxygen sensor is used to measure the concentration of the same, then repeated after five days of incubation. The difference between the two values reveals BOD5, measured in mg/l.
Then there is the COD (acronym of Chemical Oxygen Demand), which instead measures the amount of oxygen needed to degrade by oxidation all organic or mineral substances. Therefore, unlike BOD5 which measures biodegradable pollution, COD also detects non-biodegradable pollution.
The COD/BOD5 ratio allows to measure the biodegradability of an effluent:
- 2: easily biodegradable effluent
- between 2 and 4: average biodegradable effluent
- more than 4: hardly biodegradable effluent
Specific chemicals such as potassium dichromate (K2Cr2O7) in sulfuric acid (H2SO4) are used to make this measurement. The method requires two hours to get a clear picture of the activity of a purification plant. It is also used to calculate tariffs and decide the limit values for the discharge of water into the environment.
As an alternative to BOD5 it is possible to remember the use of manometric respirometers, the UV measurement that is mainly used for drinking water production and finally the exploitation of fluorescent bioreagents, whose possible survival or disappearance gives results in a couple of days.
